This term came about through a very popular cartoon of a helmet topping off a pair of boots to signify the veteran with just a short time left in his enlistment and keeping a REALLY low profile, turtle-like, trying to make it out alive. Among VietNam and older veterans a challenge term by which infantrymen recognized each other.

When a veteran infantryman would meet another veteran, the question would be asked: "Are you a turtle?". To which the challenged veteran would have to either reply "You bet your sweet ass I am!" or have to buy the challenging veteran a drink.

The Urban Dictionary T-Shirt

The art of taking a backpack, removing all items from the inside, turning it inside out, placing the items back in, along with the straps, and then closing it. The final result closely resembles a turtle shell. The key to this act is timing, and when the person who has been turtled later comes across his/her bag, the look on his/her face will tell you that you have succeeded. This act can be performed purely for the sake of entertainment, or as an act of revenge.

Wow he left his backpack in the bathroom, he definitely deserves to be turtled.